23 "As my Father has sent me," says our Lord, "even so send I you. And what He had said this, He breathed on them, and says unto them, Receive the Holy Ghost. Whose soever sins ye remit, they are remitted unto them; and whose soever sins ye retain, they are retained." Therefore, if they represented the Church, and this was said to them as to the Church herself, it follows that the peace of the Church looses sins, and estrangement from the Church retains them, not according to the will of men, but according to the will of God and the prayers of the saints who are spiritual, who "judge all things, but themselves are judged of no man." For the rock retains, the rock remits; the dove retains, the dove remits; unity retains, unity remits. But the peace of this unity exists only in the good, in those who are either already spiritual, or are advancing by the obedience of concord to spiritual things; it exists not in the bad, whether they make disturbances abroad, or are endured within the Church with lamentations, baptizing and being baptized. But just as those who are tolerated with groanings within the Church, although they do not belong to the same unity of the dove, and to that "glorious Church, not having spot or wrinkle, or any such thing," yet if they are corrected, and confess that they approached to baptism most unworthily, are not baptized again, but begin to belong to the dove, through whose groans those sins are remitted which were retained in them who were estranged from her peace; so those also who are more openly without the Church, if they have received the same sacraments, are not freed from their sins on coming, after correction, to the unity of the Church, by a repetition of baptism, but by the same law of charity and bond of unity. For if "those only may baptize who are set over the Church, and established by the law of the gospel and ordination as appointed by the Lord," were they in any wise of this kind who seized on estates by treacherous frauds, and increased their gains by compound interest? I trow not, since those are established by ordination as appointed of the Lord, of whom the apostle, in giving them a standard, says, "Not greedy, not given to filthy lucre." Yet men of this kind used to baptize in the time of Cyprian himself; and he confesses with many lamentations that they were his fellow bishops, and endures them with the great reward of tolerance. Yet did they not confer remission of sins, which is granted through the prayers of the saints, that is, the groans of the dove, whoever it be that baptizes, if those to whom it is given belong to her peace. For the Lord would not say to robbers and usurers, "Whose soever sins ye remit, they shall be remitted to him; and whose soever sins ye retain, they shall be retained." "Outside the Church, indeed, nothing can be either bound or loosed, since there there is no one who can either bind or loose;" but he is loosed who has made peace with the dove, and he is bound who is not at peace with the dove, whether he is openly without, or appears to be within.
24. But we know that Dathan, Korah, and Abiram, who tried to usurp to themselves the right of sacrificing, contrary to the unity of the people of God, and also the sons of Aaron who offered strange fire upon the altar, did not escape punishment. Nor do we say that such offenses remain unpunished, unless those guilty of them correct themselves, if the patience of God leading them to repentance give them time for correction.
Source: On Baptism, Against the Donatists (New Advent)